One known type of air-fuel ratio control system for an automotive engine regulates an air-fuel ratio to the leanest extent considerably close to a critical air-fuel ratio at which misfiring may possibly occur so as to keep the degree of operating instability of the automotive engine less than an allowable degree of operating instability. The term "operating instability" as used herein shall mean and refer to the degree of change in engine speed or in engine vibration. In the air-fuel control system, a characteristic curve of operating instability relative to an air-fuel ratio for an automotive engine is experimentally provided so as to determine the leanest air-fuel ratio in a range wherein the automotive engine operates with an allowable degree of operating instability. Such an air-fuel ratio control system is known from, for example, Japanese Patent Publication No. 56(1981)-33571 entitled "Air-Fuel Ratio Control System For Internal Combustion Engine" published Aug. 4, 1981.